Technological advancements have contributed to an increased popularity in remotely piloted vehicles (RPVs) which may be used for recreational purposes. Some of these RPVs may include flying vehicles such as planes and multi-rotor flying vehicles, more commonly referred to as “drones”. Many drone RPV systems may include First Person View (FPV) where the human operator controlling the drone, otherwise known as the “pilot, views the surroundings as if actually sitting on the drone.
A typical FPV system is schematically shown in the block diagram of FIG. 1. FPV system 100 may include a FPV drone 102 with a video acquisition system 104, goggles 106 which may be worn by the pilot and which may display video received from the drone's video camera, and a remote control unit 108 which is used by the pilot to control the drone's operation. The video from video acquisition system 104 may be wirelessly transmitted by means of a transmitting station (XMIT) 110 on the drone to a receiver (RCV) 112 which may be installed on goggles 106 and may allow the pilot to view the video real-time. The remote control unit may be a handheld unit, and may include a transmitting station (XMIT) 114 to wirelessly transmit the control signals to a receiver (RCV) 116 in the drone. Drone 102 may additionally include a controller 117 which may control all video acquisition and transmitting functions, as well as all flight control functions.
The wireless communications link in the FPV system may be considered bi-directional, with the video transmitted in a downward direction from drone 102 to receiver 112 in goggles 106, and the control signals in an upward direction from remote control unit 108 to the drone. For convenience hereinafter, the communications link may be referred to as “downlink” and may be represented by arrow 118 for the downward direction, and “uplink” for the upward direction represented by hatched arrow 120. Generally, it may be desired that uplink 120 and downlink 118 share the same radio channel instead of using separate channels as this may allow for a substantial reduction in the amount of hardware required, thereby potentially resulting in savings in system cost, weight and size. Furthermore, using the same radio channel may save spectrum resources compared to using two channels, which is also potentially advantageous as spectrum resources are generally quite limited.
FPV systems using the same channel for the uplink and the downlink may rely on a channel access method known as TDMA (Time Division Multiple Access) where the channel is divided into different time slots during which the transmitting stations may transmit. These time slots may be used by the transmitting station in the drone for downlink communications and by the transmitting station in the remote control unit for uplink communications. A MAC (multiple access controller) may coordinate the transmissions between the respective stations to reduce possible transmission collisions as a result of both stations transmitting at the same time. The MAC may use one of two possible functional configurations, a central management configuration or a distributed management configuration. With the central management configuration, only one of the stations (i.e., in the drone or in the remote control unit) may manage the TDMA by allocating the time slots to all the stations. With the distributed management configuration, each station may manage its own slots and may decide when to transmit.
FPV systems commonly use Wi-Fi as a communications scheme between the different elements in the system (i.e. goggles, controller, and drone). In Wi-Fi, the mechanism for central management is known as PCF (Point Coordination Function) while that for distributed management is known as DCF (Distributed Coordination Function). PCF is generally an optional function and is not supported by all Wi-Fi devices. DCF, on the other hand, is mandatory for all Wi-Fi devices and is therefore supported by all. Consequently, for FPV systems, use of the distributed management configuration may be more commonly used than the central management configuration.
The DCF may be based on CSMA/CA (Carrier sense multiple access with collision avoidance) and may include use of a binary exponential backoff. CSMA/CA is a protocol which requires a station wishing to transmit to listen for the channel status for a distributed inter-frame space (DIFS) interval. If the channel is found busy during the DIFS interval, the station should defer its transmission to avoid collisions. DCF may also specify a random backoff (e.g. binary exponential), which may force a station to defer its access to the channel for an extra period of time. The length of the backoff period may be randomly generated by each station. In case several stations wish to transmit (right after the termination of the current transmission) the station which randomly picked the lowest backoff may start transmitting. Stations which lost the opportunity to transmit, (since one of the other stations started transmitting before), may not continue the backoff count from the point where it stopped, rather they may have to randomly draw a new backoff value and start the count from the beginning.
FIG. 2 schematically illustrates a functional block diagram of a FPV drone 202 using same channel bidirectional communications, as generally known in the art. Components in drone 202 which are functionally similar to those shown in drone 102 of FIG. 1 are identified by the same reference numbers. Drone 202 may include video acquisition system 104, a transmitting station (XMIT) 210 including an internal buffer 103 (internal transmit queue), a receiver (RCV) 116 and a controller 117.
In operation, video data 105 from video acquisition system 104 may be transferred to internal transmit queue 103 in transmitting station 210. Video data 105 may include real-time data acquired by the video camera in video acquisition system 104, and may be compressed and/or otherwise encoded. Video data 105 may be temporarily stored in internal transmit queue 103 pending TDMA transmission by transmitting station 210 of the video data using DCF with random backoff or other access control. Transmitting station 210 may use any known wireless communication method suitable for transferring video data 105 to the remote control unit (e.g. remote control unit 108 in FIG. 1) and may include, for example, WiFi or other communication systems, for example a communication system which complies with European regulation ETSI EN 301 893 v1.8.1. Receiver 116 may receive flight control and other control data from the remote control unit and may transfer the data, represented by arrow 109, to controller 117. Similarly to transmitting station 210, the control data may be sent by the remote control unit using wireless TDMA transmission.
FIG. 3 schematically illustrates a functional block diagram of an improved RPV drone 302 using same channel bidirectional communications, as generally known in the art. Drone 302 may include video acquisition system 104, transmitting station 210 including internal buffer 103 (internal transmit queue), receiver 116 and controller 117 similar to drone 202 shown in FIG. 2. Additionally, drone 302 may include an intermediate buffer 307, a video data dump 322, and a downlink control unit 324.
In operation, drone 302 may operate similarly to drone 202 with the added advantage that video data 105 may be temporarily stored in intermediate buffer 307. This may relieve loading of internal transmit queue 103 while transmitting station 210 is waiting for a TDMA slot to transmit. If during the random backoff process internal transmit queue 103 is filled and transmitting station 210 is unable to gain a transmission slot, DL control unit 324 may divert new video data from video acquisition 104 to dump 322 until transmitting station 210 gains a transmission slot and transmits the video in internal transmit queue to the remote control unit.